You can use round() expression to snap to grid. For example:
Sprite On Drop : Sprite set X to clamp(round(self.x/100)*100, 300, 700)
This will snap the sprite between X 300 and 700, at 100px intervals.
Another option is to put invisible placeholder objects to where you want pictures to be placed. In "On Drop" event pick the nearest placeholder and set the sprite to its position.